synthon for the 14-electron fragment [Ir(L-C,C)I2]+ due to
reversible coordination of the wingtip groups.13
CSD2009-00050 CONSOLIDER INGENIO-2010, and
CTQ2011-27593 projects, and ‘‘Ramon y Cajal’’ (P.J.S.M.) and
´
We have observed that, in the 1H DOSY spectrum,14 the
peak of residual acetone shows a higher diffusion coefficient
for a sample of acetone-d6 (Dc = 4.323 ꢂ 10ꢁ9 m2 sꢁ1) than
for a solution of 3 in acetone-d6 (0.03 mmol in 0.5 mL of
acetone-d6; Dc = 3.875 ꢂ 10ꢁ9 m2 sꢁ1). This behaviour can be
ascribed to an interaction between 3 and the acetone molecules that
results in a reduction of their mobility, which could be interpreted
as a consequence of a certain degree of dissociation of the ether
functions, an essential requisite to generate the coordination
vacancies needed for the catalytic reaction to happen.
‘‘Juan de la Cierva’’ (M.I.) programmes, and the DGA/FSE
(project E07).
Notes and references
1 M. Albrecht, R. H. Crabtree, J. Mata and E. Peris, Chem.
Commun., 2002, 32.
2 M. Albrecht, J. R. Miecznikowski, A. Samuel, J. W. Faller and
R. H. Crabtree, Organometallics, 2002, 21, 3596.
3 S. H. Strauss, Chemtracts: Inorg. Chem., 1994, 6, 1.
4 (a) J. P. Collman, L. S. Hegedus, J. R. Norton and R. G. Finke,
Principles and Applications of Organotransition Metal Chemistry,
University Science, Mill Valley, CA, 1987; (b) A. Binobaid,
M. Iglesias, D. Beetstra, A. Dervisi, I. Fallis and K. J. Cavell,
Eur. J. Inorg. Chem., 2010, 5426.
5 N. M. Scott, R. Dorta, E. D. Stevens, A. Correa, L. Cavallo and
S. P. Nolan, J. Am. Chem. Soc., 2005, 127, 3516.
6 (a) P. Braunstein and F. Naud, Angew. Chem., Int. Ed., 2001,
40, 680; (b) D. S. McGuinness and K. J. Cavell, Organometallics,
2000, 19, 741.
The second step of the catalytic cycle requires coordination
of the alkyne to give 7. For this to occur, we postulate
substitution of an iodide by the corresponding alkyne, which
we expect to be the rate limiting step. This assumption is
supported by the fact that a significant reduction of reaction
rate was observed upon addition of excess NaI to the reaction
mixture. The TOF1/2 (hꢁ1) decreases from 1250 to 706 in the
case of the hydrosilylation of phenylacetylene with Ph2MeSiH.
In order to explain the b-(Z) selectivity, a 2,1-insertion of
the alkyne into the Ir–Si bond according to the modified
Chalk–Harrod mechanism15 must take place. Therefore,
invoking a metal-assisted isomerisation,10d,16 the cavity about the
two coordination vacancies stabilises 8b against 8a through
geometrical control exerted by the ligand system over the
Ir–alkenyl intermediate (Scheme 2). This assumption is supported
by the fact that the relative free energies of the postulated
intermediates 8a and 8b, calculated by DFT (B3LYP) methods
based on the model [Ir(L-C,C)(CPhQCHSiMe3)(I)(H)]BF4, show
that the latter is more stable (DG = 4.9 kcal molꢁ1).17 Finally, the
last step of the proposed Ir(III)/Ir(V) catalytic cycle would be
the reductive elimination of the b-(Z)-vinylsilane from 8b and
coordination of the iodido ligand. In this context, it seems to be
interesting to mention that there is increasing support for the
existence of Ir(III)/Ir(V) cycles in iridium catalytic chemistry.18
The proposed catalytic cycle rests on the following experi-
mental observations and DFT calculations: (i) presence of 3
throughout the reaction and hints of an equilibrium between 3
and 4, (ii) reduction of the reaction rate upon addition of NaI,
and (iii) the unusual selectivity of the reaction, only explainable
by a 2,1-insertion of the alkyne into the M–Si bond followed
by reductive elimination of the vinylsilane from 8b, which lies
4.9 kcal molꢁ1 below 8a.
7 Crystal data for 3: [C16H26BF4I2IrN4O3], orthorhombic, Pna2(1), a =
19.5550(14) A, b = 10.6594(8) A, c = 11.3757(8) A, Z = 4, Mr =
855.22 g molꢁ1, V = 2371.2(3) A3, Dcalcd = 2.396 g cmꢁ3
,
l(Mo Ka) = 0.71073 A, T = 100 K, m = 8.291 mmꢁ1, 24968
reflections collected, 6159 observed (Rint = 0.0567), R1(Fo) =
0.0252 [I > 2s(I)], wR2 (Fo2) = 0.0650 (all data), GOF = 1.023.
CCDC 883047.
8 (a) C. L. Lund, M. J. Sgro and D. W. Stephan, Organometallics,
¨
2012, 31, 580; (b) I. Ozdemir, S. Demir, B. C¸ etinkaya, L. Toupet,
R. Castarlenas, C. Fischmeister and P. H. Dixneuf, Eur. J. Inorg.
Chem., 2007, 2862; (c) C. L. Lund, M. J. Sgro and D. W. Stephan,
Organometallics, 2012, 31, 802.
9 (a) E. Langkopf and D. Schinzer, Chem. Rev., 1995, 95, 1375;
(b) J. Bergueiro, J. Montenegro, F. Cambeiro, C. Saa and
S. Lopez, Chem.–Eur. J., 2012, 18, 4401; (c) J. Montenegro,
J. Bergueiro, C. Saa and S. Lopez, Org. Lett., 2009, 11, 141.
´
´
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10 (a) E. Mas-Marza, M. Poyatos, M. Sanau and E. Peris, Inorg.
Chem., 2004, 43, 2213; (b) Y. Na and S. Chang, Org. Lett., 2000,
2, 1887; (c) M. Nagao, K. Asano, K. Umeda, H. Katayama and
F. Ozawa, J. Org. Chem., 2005, 70, 10511; (d) R. S. Tanke and
R. H. Crabtree, J. Am. Chem. Soc., 1990, 112, 7984;
(e) V. S. Sridevi, W. Y. Fan and W. K. Leong, Organometallics,
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Complexes in Organic Synthesis, ed. L. A. Oro and C. Claver,
Wiley-VCH, Weinheim, 2009, ch. 14, pp. 349–352.
11 J. Y. Corey, Chem. Rev., 2011, 111, 873.
12 For examples of Ir(v) silyl complexes see: (a) M.-J. Fernandez and
P. M. Maitlis, Organometallics, 1983, 2, 164; (b) T. M. Gilbert,
F. J. Hollander and R. G. Bergman, J. Am. Chem. Soc., 1985,
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F. J. Fernandez-Alvarez, A. M. Lo
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guez, Organometallics, 1996, 15, 823; (d) M. A. Esteruelas,
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´
pez, E. Onate and P. Ruiz-Sanchez,
´
In summary, we have developed a straightforward synthetic
route to a cationic bis-NHC iridium(III) complex (3) with two
latent coordination sites accessible under catalytic conditions.
As a first application, we have shown that the cavity created by
dissociation of the side arms leads to unusually high b-(Z)
selectivities in hydrosilylation of terminal alkynes under mild
conditions. Remarkably, the X-ray structure of 3 represents
the first example of a heterotopic Ir(NHC) complex featuring
coordinated ether functions. Steric and electronic tuning of the
ether functions in order to optimise the catalytic activity and a
deeper mechanism insight are currently on-going.
13 D. T. Tilley and B. V. Mork, J. Am. Chem. Soc., 2004, 126, 4375.
14 S. Cozzolino, M. G. Sanna and M. Valentini, Magn. Reson. Chem.,
2008, 46, S16–S23.
15 A. J. Chalk and J. F. Harrod, J. Am. Chem. Soc., 1965, 87, 16.
16 (a) I. Ojima, N. Clos, R. J. Donovan and P. Ingallina, Organometallics,
´ ´ ´
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V. Gierz, F. J. Lahoz and L. A. Oro, Organometallics, 2008,
27, 224.
17 Coordination of the outcoming iodido ligand to 8a or 8b has been
discarded based on DFT (B3LYP) calculations.
18 For examples of Ir(III)/Ir(V) catalytic cycles see: (a) J. Mazuela,
P. O. Norrby, P. G. Andersson, O. Pamies and M. Dieguez, J. Am.
´
Chem. Soc., 2011, 133, 13634; (b) K. H. Hopmann and A. Bayer,
Organometallics, 2011, 30, 2483; (c) T. L. Church, T. Rasmussen
and P. G. Andersson, Organometallics, 2010, 29, 6769.
This work was supported by the Spanish Ministry of Economy
and Competitiveness (MINECO/FEDER) (CTQ2010-15221,
c
9482 Chem. Commun., 2012, 48, 9480–9482
This journal is The Royal Society of Chemistry 2012